1. Field of the Invention
The present invention relates to a method for reproducing a gradation image and an image forming apparatus therefor.
2. Description of Prior Art
As personal computers become popular, electrophotographic full-color printers starts to gain popularity not only in business use but also in home use. In an electrophotographic printer, a dither method is generally used to reproduce a gradation image.
FIG. 1A illustrates the dither method. Referring to FIG. 1A, a density value of each pixel of inputted image data 81 is compared one to one with a threshold value of each corresponding element in a matrix MT11 which serves as a gradation pattern. When the density value of a pixel is greater than the threshold value of the corresponding element, a position corresponding to the element has a value xe2x80x9c1xe2x80x9d. Otherwise, the position corresponding to the element has a value xe2x80x9c0xe2x80x9d. As a result, a gradation pattern image 83 is obtained wherein black positions represent xe2x80x9c1xe2x80x9d. It is to be noted that the density values (levels) of the image information 81 are all xe2x80x9c4xe2x80x9d in this example.
In the threshold value matrix MT11 shown in FIG. 1A, an image with 16 gradation levels can be reproduced. Further, if each element is halved, as shown in FIG. 1B, the number of the gradation levels can be increased twice by controlling a laser emission time.
In another dither method, as shown in FIGS. 2A through 2H, output patterns are prepared beforehand in correspondence to each density value as look-up tables (LUTs). when image data is received, an image is outputted by referring the density value thereof to the look-up table according.
For example, if an image is formed with either of the above-mentioned dither methods by using the same threshold value matrix MT, images of two colors of black and magenta shown in FIGS. 3A and 3B are superposed on each other. However, there may be nonuniformity in distance between outputted dots or a change in the printing position due to the influence of pitch irregularity or the like caused by rotational irregularity of the photoconductor or the like for electrophotographic process. In such case, dots of the two colors are liable to be completely superimposed on each other, as shown in FIG. 3C or to be arranged in parallel to each other as shown in FIG. 3D. The resulting color becomes blackish in FIG. 3C, and it becomes reddish in FIG. 3D. Therefore, when even an image having a uniform density distribution is printed, a nonuniform color image is formed due to the nonuniformity in the overlap state of the dots.
Therefore, in order to prevent color nonuniformity, the overlap state of the dots is proposed to be dispersed. That is, a gradation pattern (referred to also as density pattern or exposure pattern) is prepared for each color component to change the screen angle of the gradation pattern for each color component.
For example, a unit threshold value matrix MT13 shown in FIG. 4A is used in the dither method, and it is arranged so as to be displaced in the longitudinal direction and in the transverse direction, to form a gradation pattern with a particular screen angle. In the gradation pattern shown in FIG. 4B, the unit threshold value matrices MT13 are arranged so as to be displaced in the rightward direction by xe2x80x9caxe2x80x9d pixels, or four pixels in this example, and in the downward direction by xe2x80x9cbxe2x80x9d pixels, or one pixel in this example. Therefore,
tan xcex81≈1/4,
and this means that the screen angle xcex81 is about 14 degrees. On the other hand, the gradation pattern shown in FIG. 4B is also equivalent to an arrangement in which the unit threshold value matrices MT13 shown in FIG. 4A are displaced by one pixel in the leftward direction and four pixels in the downward direction. Therefore,
tan xcex82=4/1,
and this means that the screen angle xcex82 is about 104 degrees. That is, the gradation pattern has two screen angles xcex81 and xcex82 different from each other by 90 degrees.
The resolution (the number of lines) of the image outputted by using the gradation pattern is determined so as to be in inverse proportion to a length c (unit: pixel) of a line segment which connects the centers of adjacent unit threshold value matrices MT13 and to be approximately identical for each colors. Therefore, the resolution is expressed by c in the present specification.
If a difference in screen angle is small between arbitrary two gradation patterns corresponding to each color, a texture (rosette) becomes conspicuous as shown in, for example, FIG. 5 wherein circles represent first color and triangles represent second color. Therefore, it is required to set the difference in screen angle between two colors as large as possible.
However, it is a problem that the number of elements in the threshold value matrix as a unit is limited to a number not greater than a predetermined number. Therefore, the screen angle which can be set is limited to a limited number of discrete values. Furthermore, if the resolution is increased, the number of the elements is further limited, and the value of the screen angle that can be set is further limited.
Furthermore, as described above, the gradation pattern PSG has two screen angles xcex81 and xcex82 different from each other by 90 degrees. Therefore, the screen angles that can be set for the gradation patterns for the color components are further limited.
Then, even if screen angles for the color components are varied between gradation patterns, the difference in screen angle between them cannot be increased so largely. Normally, in order to suppress color nonuniformity, it is desirable to set the difference in screen angle not smaller than 20 degrees between gradation patterns. However, when a full color is reproduced with four colors of yellow, magenta, cyan and black, the angle is insufficient. For this reason, the occurrence of color shift or color nonuniformity cannot be sufficiently suppressed or a ring-like texture (rosette) is liable to occur periodically, to degrade the image quality.
An object of the present invention is to provide a gradation reproducing method and an image reproduction apparatus capable of suppressing color shift or color nonuniformity and forming a color image or a multi-color image of which the texture is less noticeable.
In one aspect of the invention, in order to reproduce a gradation image of a plurality of colors, a plurality of patterns including first and second patterns are provided. The first pattern makes dots in the reproduced image grow as lines as gradation level increases, and the second pattern makes the dots in the reproduced image grow as lumps as gradation level increases. One of patterns for a color of the received image data is selected, and received multi-level image data are screened with the selected pattern to generate bi-level image data. The image can be reproduced with dots according to the bi-level image data.
In a second aspect of the invention, in order to reproduce a gradation image of a plurality of colors, a plurality of patterns including first and second patterns are provided, and the first and second patterns make dots to be reproduced grow discretely along lines at low gradation levels as the gradation level increases. Screen angles of the first and second patterns are different by 90xc2x0 from each other. One of the patterns is selected based on a color of received image data, and the received image data are screened with the selected patterns to generate bi-level image data for reproducing the image with dots.
An advantage of the invention is that image quality of a full-color or multi-color reproduction image is improved by suppressing color shift or color nonuniformity.
Another advantage of the invention is that even when a printing position displacement occurs, the simultaneous collapsing of spaces is hard to occur.
A third advantage of the invention is that an image is reproduced stably.